By catalyzing hydrolysis of cAMP and cGMP, cyclic nucleotide phosphodiesterases (PDEs) are critical regulators of intracellular concentrations of, and biological responses mediated by, cyclic nucleotides, including immune/in- flammatory responses. Understanding cellular regulation of PDE isoforms [which belong to seven gene families (PDE1-7)] will be of increasing importance for targeting specific PDEs in treating pulmonary disorders. Although individual cells usually contain representatives of several PDE gene families, little is known of signalling pathways involved in cytokine and growth factor regulation of different PDEs in a single cell. Incubation of FDCP-2 myeloid cells with Interleukin-4 (IL-4) increased both PDE3 and PDE4 activities, whereas IL-3, Granulocyte/Macrophage Colony Stimulating Factor (GMCSF) and phorbol ester (PMA) increased PDE4 only. IL-4 (not IL-3, GMCSF, PMA) induced rapid tyrosine phosphorylation of Insulin-receptor Substrate-2 (IRS-2 is a docking protein which functions to localize and coordinate interactions between receptors and signalling molecules/complexes). IL-4 induced activation of PDE3, PDE4 and the downstream protein kinases, mitogen-activated kinase (MAPK) and PKB, (but not IL-4 induced phosphorylation of STAT-5) were blocked by Tumor Necrosis Factor alpha (TNFalpha) which reduced IL-4-induced tyrosine phosphorylation of IRS-2 and its association with phosphatidylinositol 3-kinase (PI3-K) and by wortmannin, a PI3-K inhibitor. Downstream of PI3-K, IL-4 signals diverge, with PDE3 regulated by MAPK-independent and PDE4 by MAPK-dependent pathways. IL-3, GMCSF, and PMA also regulate PDE4 by MAPK dependent pathways. Whereas IL-3 induced activation of MAPK may be mediated by PI3-K (independent of IRS-2 signals) the effect of PMA is dependent on Protein kinase C signals. Thus, PDE4 may be regulated by MAPK activated by different mechanisms, i.e., IRS-2/PI3-K (IL-4), PI3-K ( IL-3, GMCSF) and Protein kinase C (PMA). It is possible that IRS-2 is intimately involved in IL-4 induced activation of PDE3, perhaps by initiating a specific set of signals or by targeting a specific signalling complex to PDE3. Although PDE4 is thought to be the predominant PDE isoform in most inflammatory/immune cells, PDE3 and PDE4, which exhibit a high affinity for cAMP, are both present in lymphocytes and macrophages. However, the proportion of PDE4 relative to PDE3 was greater in T-cells infected with the human T-cell leukemia/lymphotropic virus Type I (HTLV-1) than in other T cell-derived cells, including cells infected with the related retrovirus HTLV-2, NK lymphocytes, Kit and Jurkat leukemia cell lines, CD4+ T-cell clones specific sensitized to myelin basic protein. Incubation of some, but not all, HTLV-1 infected cell lines with the scr-kinase inhibitor herbimycin dramatically reduced PDE activity; the mechanism(s) and significance of this differential inhibition by herbimycin is not understood. After 5-7 days in culture elutriated peripheral human monocytes adhere to plastic surfaces and increase dramatically in size. The morphologic change is accompanied by a relative increase in PDE3B mRNA and activity, Glut 5 transporter and PDE1C mRNAs and a relative decrease in PDE4 activity and PDE4B mRNA. It will be important to define specific PDE isoforms present in alveolar macrophages and to determine if cellular inflammatory responses can be altered by PDE inhibitors.